The present invention is directed to a surgical joint replacement device; particularly, a replacement for a degenerated or ruptured disk between consecutive vertebrae in the spine.
Currently there are approximately 60,000 lumbar spine fusions performed in the United States and 30,000 lumbar fusions performed in Canada each year. Spinal fusion is frequently used as a treatment for low back pain and intervertebral disk degeneration, and the use of internal fixation has increased the ability of a surgeon to obtain a solid fusion. There is increased concern, however, that the biomechanical rigidity of the fusion and internal fixation may predispose adjacent spinal motion segments to rapid deterioration. Long-term follow up of patients undergoing a successful fusion indicates that 50 percent will continue to have complaints of pain. As in other joints, alternatives to fusing a spinal motion segment have inherent advantages.
Researchers have attempted to design a successful intervertebral disk arthroplasty device for years. U.S. Pat. No. 4,946,378 discloses an artificial disk having a pair of end bodies with a medical synthetic polymeric intermediate member held between the end bodies. The intermediate member apparently provides some flexibility. Somewhat similarly, U.S. Pat. No. 5,002,576 discloses an artificial disk having end cover plates separated by a closed corrugated tube which is filled with a viscoelastic material, like a body-compatible silicone. Also, U.S. Pat. No. 4,932,975 shows a vertebral prosthesis having a pair of end members that house suspension plates surrounded by an elastomeric medium. The end housings are interconnected with flexible and expandable bellows.
Other approaches are shown in U.S. Pat. Nos. 4,349,921, 4,714,469, 4,759,769, 4,863,476, 4,936,848, 4,997,432, 5,047,055, 5,071,437 and PCT Patent WO 92/14423. U.S. Pat. No. 4,349,921 discloses an artificial disk having convex superior and inferior surfaces corresponding to the adjacent vertebrae surfaces and being formed from two components to allow flexion and extension between the two components. U.S. Pat. No. 4,714,469 discloses a single member artificial disk having a predetermined thickness. U.S. Pat. No. 4,759,769 discloses an artificial disk having upper and lower members hinged together at a rear portion and biased apart at a front portion by stiff coil springs. U.S. Pat. No. 4,863,476 discloses a two portion spinal implant that is expandable so as to increase the spacing between the adjacent vertebrae. U.S. Pat. No. 4,936,848 shows an artificial disk having a spherical shape that is hollow and rigid. The sphere wall contains fenestrations, open to the sphere cavity, for placing bone fragments therein. U.S. Pat. No. 4,997,432 shows an artificial disk having plates separated by a sliding core body normally consisting of a synthetic material. U.S. Pat. No. 5,047,055 discloses an artificial disk made from a hydrogel material having a specified compressive strength and, when hydrated, having the shape of a human disk. U.S. Pat. No. 5,071,437 shows an artificial disk having two rigid end-plates separated by, and connected to, an elastomeric core material having flexure properties similar to those of a human disk.
Finally, U.S. Pat. Nos. 4,595,663, Re. 32,449 and 5,037,438 disclose the use of ceramic material, including zirconia, for applications such a joint replacement.
There are certain basic criteria a successful intervertebral disk arthroplasty device must fulfill. Fatigue strength of the materials is of utmost importance. Since the average age of patients undergoing spinal fusion is 42 years old, the life span of the device should exceed 40 years. Assuming the average person experiences 2 million strides per year and 125,000 significant bends in the spine, a conservative estimate of the number of spinal loading cycles over the 40-year period would be 85 million cycles. To provide a factor of safety, the device should be designed to at least a fatigue limit of 100 million cycles.
In addition to such durability, the materials for a successful intervertebral disk arthroplasty device must be biocompatible. The amount of wear of the implant must be kept to a minimum. Although the implant should be small enough to be contained within the anatomic confines of a normal disk space, it is recognized that it may be advantageous to increase the prosthetic disk height in order to over distract the disk space to unload the facet joints posteriorly.
The present invention not only satisfies these criteria, but it is anticipated that it could be a successful arthroplasty in place of 90 percent of the fusions currently being performed.